MANIPULATION OF THE MICROSCOPB. 31 



the ring widening and the center becoming sharper as 

 the objective passes downward. With the oil-bubble 

 this is reversed, the brightest center and the widest, 

 ring being above. Why this is so will be seen from 

 Fig. 18. The oil-bubble, a dense sphere in a less 

 dense medium, acts like an ordinary lens concentrating 

 light to a bright focus above, the surrounding ring from 

 which the light is diverted being dark. This dark ring 

 will widen upward. The air-bubble, on the other hand, 

 being less dense than the mucilage, diverges the rays of 

 light, and its -dark ring is widest below. 



4. The Use of the Draw-tube. — The spherical and 

 chromatic aberration of an objective vary with its rela- 

 tion to the object examined and to the eyepiece. There- 

 fore when we say that in an achromatic objective these 

 defects have been corrected, this is only true for a certa,in 

 standard set of conditions. If we change the position of 

 the draw-tube and therefore the distance between the 

 objective and eyepiece systems, a certain amount of aber- 

 ration is again introduced. For each objective there is a 

 certain proper tube-length, record of which accompanies 

 the microscope and may be placed upon the objective 

 itself. According to the best American practice the 

 tube-length, measured between the upper end of the tube 

 where the eyepiece is inserted and the lower end of the 

 tube where the objective is inserted, is either 160 or 

 216 mm. 



A cover-glass is commonly placed between the object 

 and the objective in order to hold the former in place 

 and to protect the latter; and this introduces a pertur- 



